LONDON — A racehorse owned by Britain’s Queen Elizabeth II that won the prestigious Gold Cup at Royal Ascot last year has tested positive for the banned substance morphine.

Buckingham Palace says that early indications suggest that the five-year-old filly, Estimate, consumed the substance as a result of contaminated feed.

John Warren, the queen’s bloodstock and racing adviser, said Tuesday that Estimate’s trainer Michael Stoute “is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables.”